Like others have said, accuracy is the key. Accurate cutting and piecing make the difference. I don't use glue or starch but I do pin to the background carefully. Once stitched, I cut away the background usually to reduce the bulk.
Have made several quilts using blocks hand-pieced (not very well) by a 90 year old lady. Had to redo some of the seams to make the blocks lay flat.
Another tip is to make sure your ironing board is not padded too much. You need FLAT in order to press your block flat. I use a board covered with foil, a piece of warm and natural and covered with muslin.